1. Field of the Invention
The present invention relates to an image display apparatus that utilizes a display, such as CRT (cathode ray tube) display, LC (liquid crystal) display, etc., which is provided in an information processing apparatus.
2. Description of the Related Art
An image display apparatus, which is used by an information processing apparatus, includes a video memory that stores image data which is output to a display device for image representation. Furthermore, an image display apparatus, which is used by an image-processing apparatus or image-forming apparatus, such as a facsimile, a digital copier or printer, also includes a video memory that stores image data being output to a display device for image representation.
In the video memory, if image data is stored therein as bit map data, the memory areas are wasted due to the influences of entropy redundancy contained in the bit map data. For example, the screen size of a VGA (video graphics array) monochrome display is 640 dots by 480 lines. If the white data or black data of many lines used as a background color continues and it is stored as bitmap data, the memory areas in the size of 640 dots by the number of the lines will be occupied.
Generally, the amount of information contained in image data is huge even if it is not the image data for displaying an image. If the image data is stored in a video memory by treating it in a non-compression state, the amount of the image data being stored in the video memory becomes huge. In such a case, many memory areas are needed for the storage and the cost will be increased significantly.
To avid this, when storing image data in a memory, coding compression of the image data is carried out. The amount of image data can be reduced to a certain degree by the compression, and there are various compression techniques that have been proposed to deal with image data and compression techniques combining conversion coding and arithmetic coding, such as MH coding technique, MMR coding technique, JPEG, JBIG, etc.
However, much processing time is needed on the occasion of coding or decoding in the case of the conventional coding/compression method. The conventional coding/compression method needs complicated coding processing must be accompanied with judgment processing and arithmetic operation processing. For example, a QM coder which uses an arithmetic coding technique must perform complicated processes, and the processing speed is slow.
Moreover, the conventional coding compression method is widely used in the field of image transmission by facsimile, and in order to improve the rate of compression, there is a tendency that it uses processing on the basis of page.
When taking out partially the stored image data and decoding it, or when editing the stored image data, it will be necessary to decode a large amount of image data other than the subject image data. In such a case, unnecessary processing time will be spent. The conventional coding compression method is not suitable for decoding or editing an arbitrary portion of the stored image data.
Furthermore, in the conventional coding technique which improves the degree of redundancy, the coding is performed by one-by-one calculations of run lengths, which are the length of a sequence (run) of the same data pattern, based on the pattern comparator outputs and the AND outputs of various stages. Even if it is hardware coding/decoding equipment, the processing time of a number of clocks (several clocks or several tens of clocks) is needed for outputting a run-length code.
When the conventional coding/decoding method is applied to storage of display data (image data) in a video memory, the processing time that is several tens or thousands of times as large as that of the conventional method using the bitmap data storage for editing will be needed. And the speed of image drawing will be lowered.
Moreover, in a conventional coding/decoding apparatus of a display device for binary image data, the image data is divided into data blocks each having a fixed length, and the parallel processing of pattern matching for the data blocks is carried out. The display image is stored in a video memory with the coding method which determines run-length codes and carries out the entropy coding from the matching results. At the time of decoding, the encoded image data is decoded to the original image data by the parallel processing and reverse conversion.
However, there is not any concrete image display method that can rearrange the codes indicating the run lengths quickly and store the encoded image data in a video memory. Moreover, there is not any concrete method which can discriminate the run-length code and the bitmap data.
Furthermore, the patterns which are optimal for use in the pattern matching processing are varied depending on the kind of applications. Even if the patterns are considered to be optimal for a certain application, they are not the optimal one when they are applied to another application. In such a case, the rate of compression of image data will be lowered.